The use of peroxygen bleaching agents for washing clothes and other household articles has long been known. They are particularly valuable for removing stains having a significant content of colouring matter, for instance, tea, coffee, fruit, wine and cosmetic stains. Commonly, the bleaching agent takes the form of a peroxy salt such as sodium perborate or sodium percarbonate. This is typically added to a laundry detergent composition at a level in the range from about 5% to about 35% by weight.
The effectiveness of the bleaching agent tends to be limited, however, by competing side reactions, particularly by decomposition of the bleaching agent with release of gaseous oxygen. As is well known, certain heavy metal impurities introduced into the wash process via the wash liquor, wash load or detergent ingredients can act as a catalyst for decomposition of the bleaching agent and for this reason, it is common to add a sequestering agent such as ethylenediaminetetra acetic acid (EDTA) or its salts to control the level of free heavy metal ions in solution. The effect of this under normal conditions, however, is to suppress the level of bleaching activity.
Heavy metal impurities not only catalyse decomposition of the bleaching agent, however, but they can also act to enhance the oxidizing activity of the bleaching agent if present in very small, but precisely controlled proportions. The overall objective, therefore, is to regulate the level of heavy metal ions in the wash liquor so as to provide the optimum balance of oxidizing activity and bleach decomposition.
One approach to this problem is taught in British Pat. No. 984459 wherein a combination of a copper salt and a sequestering agent having a copper dissociation constant in the range from -11 to -15, is used together with a water-soluble perborate bleaching agent. The dissociation constant of the complex is such as to provide a level of free copper ions in solution in the range necessary for activation of the perborate. Unfortunately, however, the buffering capacity of the sequestrant in this type of system is relatively weak with the result that significant variation in the level of free copper ions can still occur. Where, on the other hand, a sequestrant of greater chelating power is used, such as EDTA, the level of free heavy metal ions in solution is reduced to such an extent that activation of the bleaching agent is minimal; in other words, the bleaching agent is "overstabilized".
A generally similar approach to the problem is described in DOS No. 2,657,043 in which a preformed iron(III)/chelate complex is added to the bleaching composition. This approach depends critically, however, on maintaining equivalence of chelate and heavy metal cations with the result that the system is unable to handle the significant variations of heavy metal content introduced via the wash load or wash solution.
A further disadvantage of the above techniques is that the sequestrant operates more-or-less exclusively as an auxiliary for the heavy metal cation and becomes unavailable for other detergency functions. This is particularly important for sequestrants such as ethylene diaminetetra(methylenephosphonic acid) and diethylenetriaminepenta(methylenephosphonic acid) which, in their uncomplexed forms, have significant bleachable-stain removal capabilities in their own right, especially at low wash temperatures.
The present invention therefore provides a catalyst composition for a peroxygen bleaching agent, the catalyst composition providing improved control of bleaching activity at both low and high wash temperatures. It also provides laundry bleaching and detergent compositions having more effective and efficient usage of peroxygen bleaching agent, thereby delivering an increased bleaching performance for any given level of peroxygen bleach, or minimizing the level of peroxygen bleach for any given level of bleaching end-result performance.